Anti-human thymocyte antibodies or immunoglobulins (Igs) are known to be selective immunosuppressants. They act on the immune response by decreasing, by depletion, according to various mechanisms, the quantity of circulating lymphocytes of the blood and of the various lymphoid tissues, and probably by blocking or modulating their receptors. Their use is indicated in the context of organ transplants, for the prevention and treatment of transplant rejection, and also for the treatment of acute graft versus host reaction. These immunoglobulins have also been proposed in the treatment of medullary aplasia.
Preparations of this type of immunoglobulin were initially obtained by injection of lymphatic cells into animals, rabbits or horses for example, and then by extraction of the anti-thymocyte immunoglobulins from the immune serum. However, the level of purity of these preparations was insufficient to render these immunoglobulins usable, in particular in therapy (R. D Guttman, 1967, J. Exp. Med., 126:1099-1127). Antihuman erythrocyte or anti-basal membrane immunoglobulins were in particular produced by the animal during the immunization. Several hypotheses explain the appearance of these immunoglobulins: either the thymocyte suspensions contain traces of red blood cells (J. M. Rolland, 1972, Pathology, 4(2): 85-122), or common surface antigens exist between T cells, B cells and erythrocytes, which at the current time is the most plausible hypothesis.
Techniques were then developed in order to improve the purity of the preparation. These techniques envisioned a step of hemadsorption, namely adsorption on human red blood cells.
This step of hemadsorption makes it possible to remove the majority of the undesirable antibodies, such as the anti-erythrocyte antibodies present in the initial immune serum. A step of removal of the anti-tissue antibodies, on human tissues (such as placenta), was also sometimes envisaged, in order to adsorb the anti-basal membrane antibodies in particular. Several anti-human thymocyte immunoglobulins prepared according to these techniques are now available on the market, such as Thymoglobuline® (Imtix-Sangstat), Tecelac® (Biotest), or else ATGF® (Fresenius) or ATGAM® (Upjohn).
However, the methods described which comprise a step of adsorption on human tissues or red blood cells still exhibit drawbacks. In particular, in carrying out these methods, a 30% decrease in the level of gammaglobulins contained in the serum, and therefore a not insignificant part of the antibodies of interest, is observed. In addition, the removal of the undesirable anti-erythrocyte antibodies is not complete. Now, these antibodies would be liable to cause the appearance of hemolytic anemia in treated patients. Finally, the use of human material, such as erythrocytes, even purified and made safe, in the method for isolating the anti-human thymocyte immunoglobulins, increases the potential risk of infection.
The authors of the present invention have therefore sought to develop an improved method for producing anti-human thymocyte immunoglobulins.